FEHLING'S SOLUTION, a solution much used by chemists for the detection and estima tion of glucose and certain other sugars. In its preparation 34.64 grams of pure sulphate of copper crystals are dissolved in 200 grams of distilled water,- and (separately) 173 grams of crystallized neutral tartrate of sodium are also dissolved in 500 or 600 grams of a solution of caustic soda having a specific gravity of 1.12. The copper solution is then gradually added to the caustic soda solution, and the mixed liquid is diluted to one litre. Fehling's solution must be freshly prepared, for, like all other copper test solutions it becomes unreliable in a short time, thereafter indicating sugar where none exists. For use a small quantity of the solution is placed in a test tube and diluted with about four times its bulk of water. It is then boiled for a few seconds, and if it remains clear it may be considered to be in good condition. The fluid to be tested is then immediately added, drop by drop, until a bulk equal to that of the diluted test fluid has been added. If glucose is present a yellow precipitate of hydrated cuprous oxide is thrown down, which subsequently loses its water of hydration and beomes reduced to ordinary red cuprous oxide. When prepared as
described above, one cubic centimeter of the standard (undiluted) solution corresponds to five milligrams of glucose. Fehling's solution is of a deep blue color, and in testing a compli cated organic fluid (such as urine) for glucose it often happens that the test solution is de colorized even when no glucose is present. It is, therefore, important to observe whether or not an actual precipitate is formed, as mere de colorization is not a sufficient indication. Cane sugar does not reduce Fehling's solution in the cold, but it does so when warmed, because heat changes cane-sugar to a mixture of glucose and fructose in the presence of the alkali that the test solution contains. Maltose (malt sugar) and lactose (milk sugar) reduce Fehling's so lution. The solution is named from Hermann Fehling, who first introduced it into analytical practice. See PAW'S SOLUTION; URINARY ANALYSIS ; CARBOHYDRATES.